Mg‐MOF‐74 Derived Defective Framework for Hydrogen Storage at Above‐Ambient Temperature Assisted by Pt Catalyst

Abstract Metal–organic frameworks (MOFs) are promising candidates for room‐temperature hydrogen storage materials after modification, thanks to their ability to chemisorb hydrogen. However, the hydrogen adsorption strength of these modified MOFs remains insufficient to meet the capacity and safety requirements of hydrogen storage systems. To address this challenge, a highly defective framework material known as de‐MgMOF is prepared by gently annealing Mg‐MOF‐74. This material retains some of the crystal properties of the original Mg‐MOF‐74 and exhibits exceptional hydrogen storage capacity at above‐ambient temperatures. The MgO5 knots around linker vacancies in de‐MgMOF can adsorb a significant amount of dissociated and nondissociated hydrogen, with adsorption enthalpies ranging from −22.7 to −43.6 kJ mol−1, indicating a strong chemisorption interaction. By leveraging a spillover catalyst of Pt, the material achieves a reversible hydrogen storage capacity of 2.55 wt.% at 160 °C and 81 bar. Additionally, this material offers rapid hydrogen uptake/release, stable cycling, and convenient storage capabilities. A comprehensive techno‐economic analysis demonstrates that this material outperforms many other hydrogen storage materials at the system level for on‐board applications.

The X-ray diffraction (XRD) data of the samples are obtained using Rigaku D/max 2500, and the morphology is analyzed using scanning electron microscope SEM (JSM7500, JEOL), TEM (JEM-2100F, JEOL) and HAADF-STEM (FEI Titan Cubed Themis G2 300).Thermogravimetric analysis (TGA) and simultaneous differential scanning calorimetry (DSC) were performed using the analyzer STA-449F3 under the atmosphere of Ar.Samples were loaded and weighed in air and the ramp rates were set to 10 °C min -1 .EXAFS results were acquired at beam line 1W1B of the Beijing Synchrotron Radiation Facility.The samples were prepared as paraffin pellets, and the data fitting were further conducted by using the Athena program, Artemis program, and IFEFFIT codes.Electron paramagnetic resonance (EPR) experiments were performed on a Bruker A300-10/12 spectrometer in 297-433 K temperature range.All the EPR experiments have been performed on powder samples.Hydrogen desorption was detected using temperature-programmed desorption (TPD) spectroscopy (Autohchem II 2920), with a heating rate of 5 K min -1 and a carrier gas of N2 (flow rate of 30 cm³ STP min -1 ).The composition and elemental valence were analyzed using X-ray photoemission spectroscopy (XPS, Thermo Escalab 250Xi).The material structure and the hydrogen adsorption configurations FT-IR were examined by an infrared spectrometer (Thermo Fisher, Nicolet 6700) and NIR were examined by a near-infrared spectrometer (Thermo Fisher, Nicolet Antaris II).The precise quantity of Pt and Mg was examined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima-7000DV).The binding mode of hydrogen was detected by nuclear magnetic resonance spectrometer (NMR, Bruker AVANCE III 600 M).The magnetic field strength is 14.09 T, and the chamber cavity diameter is 54 mm.The surface area of each sample was measured by N2 sorption isotherms (SSA-7000, Builder) at 77 K using the Brunauer-Emmett-Teller (BET) method, and the porosity parameters were analyzed using the software QuadraWin (version 6.0).

Performance test
Hydrogen storage measurement.Hydrogen adsorption and desorption kinetics, cycling stability, and isothermal adsorption/desorption curves were performed using the automatic sieverts-type highpressure adsorption instrument (GASpro, Setaram, accuracy 1%R) and a manual sieverts-type pressure-composition-temperature (PCT) setup.For GASpro the tests are carried out in a sample cell volume of approximately 15.1 mL by using high-purity hydrogen gas (99.999%).The real gas state equation is applied for the calculation.The samples were loaded and weighed in air with the sample mass of 150-200 mg.Pre-treatment before test consisted of heating at 100°C for 30 min in vacuum.The sample holder volume is ~15 mL, the volume calibration was conducted using He gas for five times, and the average number is the final result.Samples were activated once by introducing 3 MPa H2 gas and holding at 250°C for 30 min, followed by evacuating for 30 min.The determination for equilibrium in PCT test was the fluctuation of less than 0.002% in pressure value within 1 min.For cyclic test, a dynamic vacuum of 30 min was arranged before each cycle of hydrogen absorption to ensure complete dehydrogenation of the sample.The standard Pd powder was used for calibration to prevent the systematic errors and influence of the temperature gradient between the sample cell (298-598 K) and gas reservoir (303 K).Enthalpy and entropy calculations.The PCT curves are fitted by using exponential fits, the differential enthalpies of adsorption (∆H) and entropies of adsorption (∆S) were calculated using the Clausius-Clapeyron relationship in equation (1), where R is the ideal gas constant, P is the pressure, and T is the temperature.
Activation energy of desorption.The Kissinger equation is applied for the calculation of the activation energy (  ).The validity of this method has been thoroughly described previously [1] and is represented as equation (2), where  is the heating rate, A is the frequency factor, R is the ideal gas constant, and   absolute peak temperature.

DFT calculation.
Pristine structure of Mg-MOF-74 with the additional solvent molecule was obtained in the Cambridge Structural Database.Upon activation, the solvent molecule will be removed, leaving a primitive rhombohedral unit cell.The defective structure was obtained by removing a linker from the unit cell.
The hydrogenated structure was obtained by adding H2 molecule or H atom randomly into the channel.All framework atoms are relaxed during the structural optimization in this work.As for the hydrogen absorption calculation, only adsorbing hydrogen are relaxed while the framework atoms are fixed because of the minor effect of the framework flexibility.The adsorption amount on one defective MgO5 site is assumed to be the same as that on the others.All geometric optimization and energy calculations were performed in the Vienna ab initio simulation program (VASP) using density functional theory (DFT).The non-valent nuclear electrons are represented by the projected augmented wave method (PAW).All spin polarization calculations were performed by the generalized gradient approximation and the Perdew-Burke-Ernzerhof (PBE) electron exchange-correlation interaction.A kinetic energy cutoff of 480 eV was chosen and the Brillouin zone was sampled in the k-space by a dense grid of 3 × 3 × 5.The atomic positions are relaxed and unconstrained completely until the maximum force on each atom is 0.01 eV Å -1 .The energy convergence criterion is set to 10 -5 eV.Grimme's DFT-D2 method is adopted for the calculation of van der Waals interactions.The adsorption energies were estimated via the equation (3): denotes total energy derived from static DFT calculations.The subscripts  +  2 /,  and  2 /denote the hydrogen adsorption system, bare perfect framework or defective framework, and free gas molecule or atom, respectively.

Techno-economic Analysis
The objective of this part is to access the techno-economic performance of the materials used for onboard hydrogen storage.A useful life of 1000 cycles and a storage capacity of 5.6 kgH2 per tank are considered. [2]Neglected are the impacts of internal mass and heat transmission within the storage tanks, as well as heat exchange and insulation between the ambient and the storage system.The amount of hydrogen stored in the tank is normalized to establish the system-level performance.Consideration includes the tank volume, vessel mass, and storage material mass.The standard energy density of hydrogen is 33.6 kWh/kg.The outcome of the economic study is provided in USD to make an easier comparison for the global market.The price of electronic energy is set at $0.067 per kWh. [3]he levelized cost of storage (LCOS) in the proposed model is made up of the four cost fractions listed below for representative promising materials: 1.The cost of tank depreciation,   ; 2. The cost of hydrogen compression,   ; 3. The cost of heat and/or refrigeration for maintaining temperature,   .4. The cost of hydrogen storage materials depreciation,   .Since the price of hydrogen gas is the same for all the storage methods under consideration, it can be disregarded for the sake of comparison in this work.The LCOS is the sum of the four fractions as determined via equation ( 4): For the tank system, it is assumed that the main cost driver is the pressure vessel.  is calculated based on the equation (5): (5) The fitted parameters of    ,    and    are derived from previous work. [4]The   , which reflects the quantity of hydrogen uptake/release cycles, shows how long a tank will last. represents each tank's storage capacity.  is set to 1000 in this work, and  is set to 5.6 kgH2.
In the hydrogen compression process, each compression step is followed by a cooling step.It is designed to keep hydrogen stored in various materials at the right temperature.  is calculated by the multiplication of the electric energy cost   (0.06 USD/kWh) and the energy demand for hydrogen processing   .The   is determined using equation ( 6) and (7): ℎ  and ℎ  represent the isentropic enthalpy of output and input hydrogen, respectively.The NIST database is used to retrieve the thermodynamic data. is the mass flow of hydrogen.The value of   , which stands for isentropic compression efficiency, is 0.9.The detailed explanation has previously been reported. [4]or heat and refrigeration, the capital cost and energy cost are determined from the storage temperature.The   for TiFeMn, rGO-nanoMg and Pt-de-MgMOF are calculated using equations ( 8) -(10).While the   for MOF-5 is calculated using equations ( 8), ( 9) and ( 11): =  0   −  0 (9) (10)   =  2.4647−0.01812× 277.778 (11)     is the ideal refrigeration cycle coefficient of performance. 0 is the lowest thermodynamic temperature of the cold side,   is the room temperature.ℎ  ′ and ℎ  ′ is the isentropic enthalpy of cooled output hydrogen and input hydrogen, respectively.  is the desired output temperature.The processes' ability to recover heat is disregarded, and it is assumed that all heat flux is lost to the environment.The detailed description of the parameters and equation can be found in previous studies. [5]n terms of storage materials, the most explored intermetallic compound, TiFeMn, is selected as the representative material because it has good storage properties. [6]Raw materials cost and gravimetric hydrogen density of TiFeMn have already been reported. [7]The cost of rGO-nanoMg is estimated using an engineering scale-up of synthesis techniques that have been demonstrated in lab. [8]Supplier prices for each of the chemical reagents are gathered.The price for MOF-5 and Mg-MOF-74 is calculated from prior research using actual production methods. [9]Price quotes for chemical reagents are collected from suppliers.  is calculated based on the equation ( 12): is the price of material in USD/kg,   is the gravimetric hydrogen density.Each   is obtained from previous studies while the   for Pt-de-MgMOF is 2.55 wt%.Hydrogen capacity (wt%)  The guest molecules of DMF have been successfully removed after the annealing at 400 °C. [10]In addition, a significant decrease in the resonance peak at 8 ppm (attributed to phenyl hydrogen) [11] indicates partial degradation of the linker.

Figure S1 .
Figure S1.TGA (black line) and DSC (navy line) curves for Mg-MOF-74 under the atmosphere of Ar.

Figure S2 .
Figure S2.Digital photographs of a series of Mg-MOF-74 after the heat treatment at the indicated temperatures.

Figure S3 .
Figure S3.Hydrogen adsorption amount of a series of thermally treated Mg-MOF-74 measured under 50 bar H2 and 200 °C.

Figure S7 .
Figure S7.(a) BET isotherm and (b) BJH pore size distributions of Mg-MOF-74.(c) BET isotherm and (d) DFT pore size distributions of de-MgMOF.The open and solid symbols represent the absorption and desorption of the isotherm, respectively.

Figure S16 .
Figure S16.XRD patterns of Pt-de-MgMOF before and after 10 cycles of hydrogen adsorption/desorption at 160 °C, with a reference of Pt nanoparticles.

Figure S17 .
Figure S17.(a) BET isotherm measured at 77 K using liquid N2, and (b) the pore size distribution of Pt-de-MgMOF after 10 cycles of hydrogen adsorption/desorption.

Figure S21 .Figure S22 .
Figure S21.Calculated entropy change of Pt-de-MgMOF as a function of the hydrogen content.

Figure S25 .
Figure S25.(a) Schematic illustration of the hydrogen adsorption on an MgO5 knot.(b) Calculated configuration and hydrogen adsorption energy (Eads) when a molecular hydrogen is absorbed on the desolvated Mg-MOF-74.

Figure S26 .
Figure S26.The charge density differences of Mg-MOF-74 and de-MgMOF are plotted with an isovalue of 0.210 -3 e Å -1 .The charge accumulation and depletion are colored in yellow and cyan, respectively.

Figure S28 .
Figure S28.Calculation results for hydrogen spillover on Pt-de-MgMOF.(a) Pt-de-MgMOF with an H2 molecular.(b) H2 molecule dissociated into two H atoms on Pt catalyst.(c) one H atom moves to the Mg atom close to the Pt.

Figure S31 .
Figure S31.Tornado chart showing parameter sensitivity of Pt-de-MgMOF in the system level.

Table S5 .
TPD results of Pt-de-MgMOF that hydrogenated at 1 bar and 160 °C.